Development within offshore oil and gas exploration in the recent years has been directed to subsea installations for processing and transport of oil and gas. These subsea installations replace the traditional platforms, where oil and gas were transported up to the platform for further processing and transport. This development of subsea production, processing and transport systems has resulted in an increasing need for tubular joints suitable for subsea use, e.g. for joining fluid conduits belonging to interconnectable processing modules or pipe sections in a fluid-tight manner. It has been realised that seals based on corrosion resistant metallic materials are more suitable than conventional elastomeric seals for use in subsea applications. One known type of tubular joint is provided with a metallic seal ring positioned between tapered inner sealing surfaces of two adjacent tubular members for sealing therebetween. Such a sealing arrangement is very effective for sealing against internal fluid pressure within the tubular members.
In subsea applications, it is often desirable to test the sealing efficiency of the metal-to-metal seal between the seal ring and the tubular members using external fluid pressure. Such testing is often termed back seal testing. This can be accomplished by providing an additional elastomeric seal between the tubular members radially outwardly of the primary metal-to-metal seal formed between the seal ring and the tubular members and introducing a pressurized test fluid into the space between the primary seal and the additional seal through a fluid channel having an outlet opening in an end surface of one of the tubular members at a location radially outwardly of said primary seal and radially inwardly of said additional seal.
Recently, there has been an increasing tendency to require double barrier seals for additional reliability of a tubular joint, both in an environmental context and in order to improve production stability. This can be accomplished by closing the above-mentioned fluid channel and utilizing said additional seal as a secondary seal. A disadvantage with this solution is that the additional seal seals at a larger diameter than the primary metal-to-metal seal. The radial area between the primary metal-to-metal seal and the additional seal will give an increased pressure area for the internal fluid pressure and thereby an increased separation force between the tubular members in case of a fluid leakage over the primary seal. Another disadvantage is the risk for hydrolocking during opening of the tubular joint, which could make it difficult or even impossible to separate the tubular members from each other. Such hydrolocking might occur when there is an underpressure, in relation to the hydrostatic pressure of the surrounding sea-water, in the space between the primary metal-to-metal seal and the additional seal.